Most magnetic materials are characterized by a relative permeability which decreases comparatively steeply when heated into a predetermined temperature range just below the Curie point of the magnetic material. In most such materials the relative permeability can again be increased by cooling the material toward the lower limit of this predetermined temperature range. Certain magnetic materials, such as certain ferrites, have a low enough Curie point and sufficient sensitivity to enable this characteristic to be conveniently and effectively utilized for effecting rapid changes in the inductance of a ballast.